1. Field of the Invention
The invention relates to a method of determining a nuclear magnetisation distribution in a part of an object where, using a sequence of at least three rf pulses in combination with different gradient magnetic fields or not, a resonance signal is generated and sampled in order to obtain a local or non-local spectrum of matter in which weakcoupled nuclear spins occur.
1. Description of the Prior Art
In a publication in Chemical Physics Letters, Vol. 69, No. 3, Feb. 1, 1980, pp. 567-570; entitled "Separation of the different orders of NMR-multiple quantum transitions by use of pulse field gradients" A. Bax discloses a pulse sequence whereby multiple quantum coherences are measured in a system of two weakcoupled spins. The described pulse sequence comprises three 90.degree. pulses where, subsequent to a first 90.degree. pulse a second 90.degree. pulse is generated after a waiting period .tau., and a third 90.degree. pulse is generated after a waiting period Tm, acquisition taking place approximately a period of time .tau. after the third 90.degree. pulse. The waiting period .tau. between the first two pulses is generally chosen to be equal to 1/(2J) but may deviate from 1/(2J), where J is the coupling constant between the two coupled spins. The effect thus obtained consists in that a maximum amplitude is reached of the zero and two quantum coherences which evolve during the period Tm between the second and the third 90.degree. pulse. For the zero-quantum coherence the evolution takes place at an angular frequency which corresponds to the difference between the chemical shifts of the coupled spins, while the evolution for the two-quantum coherence takes place at an angular frequency which corresponds to the sum of the chemical shifts. The third 90.degree. pulse will make these coherences observable in the form of a one quantum coherence signal which will be maximum after a period of time .tau. subsequent to this third pulse. When use is made of the described pulse sequence, however, the amplitude of this multiple quantum coherence signal will also be dependent on the chemical shifts, i.e. due to the choice of the waiting period .tau. between the first two 90.degree. pulses. This dependency on chemical shift can be eliminated in practice by generating a 180.degree. pulse at the instant .tau./2 during the waiting period elapsing between the first and the second rf pulse. The described pulse frequency sequence enables measurement of multiple quantum coherences, but not of discrimination between the zero and two quantum coherences.